Perfluoropolyethers having a functional group at one terminal, hereinafter referred to as “one-terminal functional derivative”, and perfluoropolyethers having functional groups at both terminals, hereinafter referred to as “both-terminal functional derivative”, hereinafter collectively referred to as “functionalized polymer”, are generally used as a precursor of various derivatives for surfactants and surface treatment agents. For instance, examples of a precursor of surfactants which are useful for polymerization include acryl derivatives, amine derivatives and isocyanate derivatives of the functionalized polymer. Further, alkoxy derivatives, chlorine derivatives and silazane derivatives of the functionalized polymer are used as a precursor of surface treatment agents.
Properties of the one-terminal functional derivatives are different from that of the both-terminal functional derivatives. For instance, the both-terminal functional derivatives cause extension of a chain or gelation, but the one-terminal functional derivatives do not cause such actions. Further, a composition comprising a polymer having no functional group at any terminal, referred to as “non-functionalized polymer”, has a problem such that the composition is not sufficiently cured. Accordingly, it is industrially important to increase a content of the functionalized polymer in a composition. In the present specification, “functional” or “functionalized” means “having a reactive functional group” and “non-functionalized” means “having no reactive functional group”.
The preparation of perfluoropolyether having the structure, —(OCF2)p(OCF2CF2)q(OCF2CF2CF2)r(OCF2CF2CF2CF2)s—, in a main chain and a functional group at one terminal is difficult. In the aforesaid structure, p and q are, independently of each other, an integer of from 5 to 300, r and s are, independently of each other, an integer of from 0 to 80, and a total of p, q, r and s is 10 to 500. Japanese National Phase Publication No. 2009-532432, Patent literature 1, describes that the preparation of a composition comprising a one-terminal functionalized perfluoropolyether in the specific content, comprising steps of preparing a mixture of a one-terminal functionalized perfluoropolyether, a both-terminal functionalized perfluoropolyether and a non-functionalized perfluoropolyether by fluorinating a part of functional groups of a both-terminal functionalized perfluoropolyether and, then, subjecting the mixture to distillation. However, in the method described in Patent literature 1, the separation is caused by making use of difference of boiling points of the components, so that the method cannot be used when the molecular weight distribution of at least one of perfluoropolyethers is wide. The upper limit of the molecular weight is such that the perfluoropolyether can be distributed. The upper limit of the average molecular weight in the Examples described in Patent literature 1 is approximately 1,000. Therefore, it is difficult to apply the method to perfluoropolyether having a larger molecular weight.
The preparation of a composition comprising a one-terminal functionalized perfluoropolyether in a higher content without any limitation on a molecular weight and a molecular weight distribution is useful for preparing materials such as surface treatment agents, lubricants, and elastomers. Therefore, development of a method for preparing a composition comprising a one-terminal functionalized perfluoropolyether in a higher content is desired.
Japanese Patent Application Laid-Open No. 2012-233157, Patent literature 2, and Japanese Patent Application Laid-Open No. 2012-72272, Patent literature 3, describe a method such that a mixture of a one-terminal functionalized polymer, a both-terminal functionalized polymer and a non-functionalized polymer, in which the content of the one-terminal functionalized polymer is large, is prepared by fluorinating a part of functional groups of a both-terminal functionalized polymer and, then, a non-functionalized polymer is removed. Patent literatures 2 and 3 state that in the step of fluorinating a part of functional group, the fluorination can be controlled by adjusting the amount of the fed fluorine gas to thereby decrease an amount of the remaining both-terminal functionalized polymer, so that, a composition comprising a one-terminal functionalized polymer in a large amount can be prepared. However, a lot of non-functionalized polymers are also formed in this method. Patent literatures 2 and 3 also describe removal of the non-functionalized polymers by adsorption with an ion-exchange resin or thin-film distillation.
Japanese Patent Application Laid-Open No. 2001-164279, Patent literature 4, describes a method for the preparation of a fluorinated lubricant used in magnetic recording media, wherein a fluorinated lubricant with an introduced functional groups, such as a piperonyl group, at the both terminals is subjected to chromatography in which a moving phase is supercritical state carbon dioxide and a stationary phase is silica gel to collect plural fractions and, then, a fraction having large amounts of a functional group is selected to thereby obtain the fluorinated lubricant having large amounts of a functional group, in particular, 95% or more, based on a total amount of terminal groups. Patent literature 4 states that the percentage of the terminal modification of the both-terminal functionalized polymer is increased from approximately 90% to 99% according to the method.